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Voltage RegulatorsReference Informatio

R225-90-2Voltage Regulators vs. Load Tap Changers

December 1999 • New IssuePrinted in USA

It appears to be rather simple: The basic function of an

electrical utility is to supply power to its customers.Voltage to be supplied is defined within a pre-determinedrange.

Unfortunately, the laws of physics enter in, with systemlosses and voltage sags that must be corrected and reg-ulated (in a technical sense) in order to stay within thevoltage range. There are choices to be made in how toregulate voltage; some of these choices can require sig-nificant capital expenditures.

For many utilities, demand for electricity continues togrow. New demand must be met with increased supply.Demand Side Management and Distribution Automationlook to utilize existing system resources more efficiently.

Paybacks from lower system losses and deferred use of costly peak demand electricity justify purchases of newequipment. Modular designs are favored, using productsoptimized for their function. Future benefits would likelybe realized in ease of repair and improved availabilitywhen replacement is needed.

Even in the real world of ongoing maintenance and occa-sional system or component failures, objectives of mini-mizing outage time and limiting equipment failures canbe realized. The use of more simple, basic, modular products increases the potential for benefits in productavailability, performance optimized in product function,and reduced concentration on large engineering design

development. Modular equipment offers the flexibility of modeling each feeder to supply service in an optimizedrange across most of the line or to the load center.

To meet today’s as well as anticipated future needs, util-ities will be focusing their efforts to better utilize equip-ment already in place, and to carefully evaluate the inclu-sion of new equipment. Three factors to consider in a

new equipment purchase decision are these:

Expected utilization level of the equipment,

Ease of use of the equipment, and

Future maintenance schedules and anticipated reparate.

 As an example, in the purchase of equipment to regulatsubstation voltage, the following product alternativemay be considered (see Figure 1):

(1)Single-phase Regulators in conjunction with aTransformer without a Load Tap Changer (LTC)

(2)Single-phase Transformer with LTC in one packag

Consider the common situation where equipment maintenance or failure requires taking product offline, i.e., deenergization. With a Transformer with LTC, wheneveone phase of the tap changer fails or requires maintenance, the entire "package" is removed from service--aphases are affected. With single-phase Regulators, one phase goes offline, it can be replaced with anothesingle-phase unit. One spare Regulator can roll througall three phases when maintenance is required; thsame spare unit can function effectively on any of thsystem’s three phases. Full maintenance can be completed without diminishing service anywhere on the system. Spare equipment may be available from emergencreserves, from another substation, or as new--direct from

the manufacturer or nearby distributor.Operation of a single-phase Regulator is essentially thsame as for a three-phase device. The main difference in the separation of the three phases into separate tankIn separating the active tap-changing mechanisms, several benefits are seen. Each single-phase unit has aindependent control to react individually to the voltag

 Alternative (1)

Transformer 

w/out LTC

 Alternative (2)

Transformer w/ LTC

TransmissionFeed

TransmissionFeed

DistributionFeeders

OvercurrentProtection

Overcurrent

Protection

1-Ph

Reg.

1-Ph

Reg.

1-Ph

Reg.

Figure 1.

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Voltage Regulators vs. Load Tap Changers

variances of the phase. Where phase voltages areunbalanced, e.g., in single-phase loading on residentialfeeders, single-phase units can respond individually tothese independent feeder models. Individual line dropsettings can be used to model each phase.

By separating the phases, each into its own unit, everyunit is made smaller and easier to handle. To a utility, thiscan mean the difference between using material-movingequipment at hand versus having to contract out for spe-cial services. Change-outs can be completed in hours,rather than days. Normal service is quickly restored.Nuisance complaints are minimized.

The Engineering function is often called upon to providea contingency plan to transfer load in the event of prod-uct failure or scheduled maintenance. Bypass switchingprovides easy installation and the ability to remove a reg-ulator without "dropping" the entire feeder or bus.Bypassing and the use of spare regulators allow for asimpler load transfer. When standard products are used,the product manufacturer can assist in devising a voltageregulation plan. This type of plan can be integrated into

other utility projects such as load management or SCADA connection at the substation.

Cost comparisons for the two alternatives are shownbelow.

TABLE 1

Life Cycle Cost Evaluation

Clearly, Alternative (1), the single-phase Regulator inconjunction with the non-LTC Transformer, is more costeffective. Further, both the Regulator and theTransformer in Alternative (1) will typically be of a morestandard design than the unit in Alternative (2).

Standardization brings additional time savings and costsavings through shorter order leadtimes, availability of lower cost spares from the manufacturer, versatility of spares in utility stock, ease of field installation and main-tenance, and increased line crew safety levels throughconsistent operation procedures.

Simplification through standardization. The voltage sup-ply function can be made simpler, after all.

© 1999 Cooper Industries, Inc.

P. O. Box 1640Waukesha, WI 53187http://www.cooperpower.com

Regulators Other Initial Maintenance every 5 years 30 yr  Equipment Cost will have 5 charges over 30 years Cost

(1) Transformer & $ 95,000 $2,500 $97,500 $10,000 each occurrence $147,5001Ø Regulators

(2) LTC transformer  $120,000 $ 0 $120,000 $35,000 each occurrence $295,000

Other equipment includes additional buswork and bypass switches.

Maintenance costs are based on $25,000 per LTC repair and a $10,000 Engineering study for load transfer when a LTC unit is

removed, $10,000 for a regulator change-out and repair.

Based on 7620 phase-to-neutral, 438 ampere rating.